Prior to the present invention, studies were made of dihaloethylene dihydric phenol polycarbonate copolymers as shown, for example, by Z. Wielgosz et al, Infrared Spectrascopic Investigation of Polycarbonates, Plaste Und Koutschuk, Vol. 19, (1972) No. 12, pages 902-904. Polycarbonate copolymers having chemically combined units derived from dihaloethylene dihydric phenol have also been utilized in the production of flame retardant rigid thermoplastic foams, as shown by copending application Ser. No. 10,496 of G. E. Niznik, filed Feb. 8, 1979 and assigned to the same assignee as the present invention. (See above). Such polycarbonate copolymers also can be found in Sannes et al U.S. Pat. 4,118,370, also assigned to the same assignee as the present invention.
It has been found that in the manufacture of polycarbonates based on the phosgenation of isopropylidene p,p'-dihydric phenol, isopropylidene-o,p'-dihydric phenol is often present as an impurity. This results from the formation of the o,p'-bisphenol-A in the manufacture of p,p'-bisphenol-A from the acid catalyzed condensation of phenol and acetone. In addition, the isopropylidene-o,p'-dihydric phenol has been found, as shown below in Table II, to act as a chain terminator causing a reduction in the molecular weight of the resulting polycarbonate. Those skilled in the art know, however, that asymmetric phenols, such as o,p'-dihydric phenol, generally effect a reduction in the glass transition temperature of the resulting polycarbonate. Accordingly, if it were possible to copolymerize an o,p'-dihydric phenol with a p,p-dihydric phenol to produce a high molecular weight polymer, the resulting polycarbonate copolymer would be expected to combine polycarbonate performance with improved processability.
Surprisingly, it has been discovered that unlike o,p'-isopropylidene dihydric phenol, o,p'-dihydric phenol of the formula, ##STR1## can be copolymerized with various dihydric phenols without significantly reducing the molecular weight of the resulting polycarbonate, while imparting improved processability and flame retardance.